Exercise weight stacks are known in the art. Generally speaking, weights are arranged in a stack and movably mounted on guide rods. A selector rod is connected to a desired number of weights by means of a pin or another suitable connection method. The selector rod and any selected weights are typically connected to a force receiving member by means of a cable which moves the weights upward in response to exercise movement.
Although exercise weight stacks are prevalent in the exercise industry, they nonetheless suffer from certain shortcomings. For example, in order to provide a large amount of weight at a reasonable cost and within a reasonable amount of space, equipment manufacturers use a small number of relatively heavy weights. As a result, the amount of weight being lifted cannot be adjusted in small increments. On the other hand, a relatively large number of lighter weights could be used in order to provide smaller increments in weight adjustment, but the resulting equipment would be quite expensive and/or bulky.
Attempts have been made to address the issue of incremental adjustments. One such effort involves the provision of a second, adjacent weight stack comprising weights which weigh a fraction of the weights in the other or primary stack. A problem with this approach is that it adds significantly to the cost of the equipment. Another effort involves the provision of a half-weight (or other fractional weight), which weighs one-half the weight of each weight in the stack, and which is selectively movable from a peg on the frame onto an aligned peg on the top plate of the stack. This approach not only creates a balance problem during movement of the selected weights, but it also increases the potential for injury due to the proximity of the two pegs and their movement relative to one another.
Yet another prior art machine with supplemental weights is disclosed in French Patent No. 2,613,237 to Louvet. The Louvet machine includes a stack of primary weights movable along a guide rod in response to exercise movement, and a stack of secondary weights movable along the guide rod and selectively stored above the stack of primary weights. The secondary weights are supported by gates which are rotatably mounted on rigid frame members and which have pegs that rotate into engagement with holes in the frame members. Each of nine secondary weights has a mass equal to one-tenth the mass of one of the primary weights.
One disadvantage of the Louvet machine is that nothing prevents a user from releasing a secondary weight without grasping the weight being released. As a result, the secondary weight may be free to drop downward onto the top plate in the stack of primary weights, thereby increasing the likelihood of personal injury and/or damage to the machine. Also, each of the secondary weights is not separately supported by a respective gate. As a result, the entire stack of secondary weights may be released at one time, with or without a user holding onto to any of the secondary weights. Yet another shortcoming of the Louvet machine is that nine secondary weights are required to provide nine levels of incremental weight adjustments.
Another limitation with many existing weight stack machines, including the Louvet machine, is that the amount of resistance is uniform throughout the range of exercise motion, whereas the user's strength typically varies as a function of muscle contraction and extension. One response to this problem has been to use eccentric cam members to vary the amount of leverage being exerted against a fixed amount of weight. However, room for other solutions and/or improvements remains.